1. Field of the Invention
The present invention relates to an apparatus for converting a voltage signal into a pulse signal with a pulse-width corresponding to the voltage level of the voltage signal, in other words, it relates to an analog-to-digital (A-D) converter for converting voltage into a pulse signal with a corresponding time width.
2. Description of the Prior Art
A double integrating circuit can be cited as a representative example of the A-D converter for converting the voltage into a time width. According to the A-D converter of this type, precision in the A-D conversion is not affected by the precision of a charge-discharge capacitor or resistor of the integrating circuit, or by variance in the frequency of clock signals. However, dielectric loss of the charge-discharge capacitor turns out to produce errors in the A-D conversion. Therefore, when the A-D converter is to be constructed in the form of a hybrid integrated circuit, the use of a chip capacitor having dielectric loss is impossible. The charge-discharge capacitor must thus be provided outside the circuit, which makes it difficult to reduce the size as a whole.
Further, the A-D converters on the market have been so constructed that the converted digital values establish a linear relation relative to input voltages, so that they can be used for general purposes. Therefore, error in the voltage that is to be converted, which corresponds to the error of one digit, increases with the decrease in voltage. When the commercially available A-D converters are to be used, therefore, it is attempted to increase the number of bits in order to increase precision in the region of low voltages. If the number of bits is increased, however, the circuit operates maintaining unnecessarily increased precision in excess of a required precision in a region of high voltages, and the manufacturing cost is increased unnecessarily.